In recent years, an optical disk has been utilized in a variety of fields of audios, videos, computers and the like, because of its capability of recording a large quantity of information signals with high density.
Particularly, optical disks having various different specifications (specs) such as CDs, CD-Rs and DVDs have been commercially available. What is required of an optical pickup is compatibility with these disks of the different specs so that a single optical pickup can perform recording or reproducing information of all types of disks.
In the case of CDs and CD-Rs, a substrate and/or a recording medium have characteristics which are optimized for an infrared light beam having a wavelength in the vicinity of 780 nm. Similarly, in the case of DVDs, such characteristics are optimized for a red light beam having a wavelength in the vicinity of 650 nm. Further, a development of a recording or reproducing disk for which the use of a blue light beam of about 400 nm would be available in future has been in progress.
An example of the optical pickup compatible with the disks for thus recording and reproducing using different wavelengths is disclosed in Japanese Unexamined Patent Publication No. 128794/1997 (Tokukaihei 9-128794 published on May 16, 1997), a configuration of which is shown in FIG. 37.
This optical pickup is provided with a first semiconductor laser 1, a second semiconductor laser 2, a three-beam diffraction grating 3, a lattice lens 4, an objective lens 5, a hologram element 7, and a light receiving element 8. The first semiconductor laser 1 starts oscillating when a wavelength of laser light is in a 635 nm band, and the second semiconductor laser 2 starts oscillating when a wavelength of laser light is in a 780 nm band. The three-beam diffraction grating 3 causes a light beam of each light sources to emerge as three beams which are used for tracking control. The lattice lens 4 acts as a concave lens depending on a direction of a polarized wave of the light beam. The hologram element 7 diffracts light reflected from a disk 6, thereby guiding it to the light receiving element 8.
Here, the first and second semiconductor lasers 1 and 2 disposed so that the directions of polarized waves thereof mutually intersect.
First, the following will explain an optical system in the case of using the first semiconductor laser 1 of the 635 nm band to play back an optical disk having a plate thickness of 0.6 mm. Light emitted from the semiconductor laser 1 is separated into three beams by the diffraction grating 3 and transmitted through the hologram element 7, thereafter simply passing through the inactive lattice lens 4 so as to converge on the disk 6 by the objective lens 5.
The light reflected at the disk 6 and returned therefrom is similarly diffracted at the hologram element 7, thereafter being guided to the light receiving element 8. The light beams in the directions of the polarized waves respectively have such lattice patterns as to be acted upon by the lattice lens 4.
Next, the following will explain an optical system in the case of using the second semiconductor laser 2 of the 780 nm band to play back an optical disk having a plate thickness of 1.2 mm.
Light emitted from the semiconductor laser 2 is separated into three beams by the diffraction grating 3 and transmitted through the hologram element 7, thereafter receiving the concave lens action of the lattice lens 4 and converging on the disk 6 by the objective lens 5.
The light reflected at the disk 6 and returned therefrom is similarly diffracted at the hologram element 7, thereafter being guided to the light receiving element 8. The light beams in the directions of the polarized waves respectively have such lattice patterns as to be acted upon by the lattice lens 4.
Note that, it is designed that the concave lens action of the lattice lens 4 corrects spherical aberration which emerges when a disk thickness is in a range of 0.6 mm to 1.2 mm.
In this arrangement, in the case of the first semiconductor laser 1 for example, the hologram element 7 is designed so that the diffraction light of disk reflection light is guided to the light receiving element 8.
Further, in the case of the second semiconductor laser 2 having another wavelength, it is arranged so that a focus point of the disk reflection light on the light receiving element 8, which tends to vary due to different diffraction angles formed by different wavelengths, is kept close at a right position.
Further, both the light from the first semiconductor laser 1 and the light from the second semiconductor laser 2 are respectively separated into three beams by the diffraction grating 3, and the same receiving element 8 detects tracking error signals according to a three-beam method.
With this arrangement, it is possible to commonly use the single light receiving element 8, two of which have been required conventionally, thereby reducing the number of components and the number of steps in the assembly.
In the case of the conventional optical pickup, with regard to semiconductor laser light having a plurality of wavelengths, it is designed that a positional relationship among light sources is set according to a predetermined value, thereby guiding the light to the shared light receiving element by the single hologram element.
However, in the case where laser and the light receiving element are integrated into one package, the laser and the light receiving element are in general fixedly located at a predetermined position, that is, a stem within the package. Therefore, it is often the case that the control of a position and/or rotation is not available for the light receiving element when controlling the hologram element.
Namely, an offset control of a focus error signal and/or tracking error signal for example, which is caused by an error in the mounting of the laser or the light receiving element, form tolerance in a phase on which the hologram element is mounted, or the like, is in most cases performed by the control of the hologram element alone. However, in that case, when optimizing the hologram element for one of semiconductor laser light sources, it is very likely that the same optimum condition becomes ineffectual when using the hologram element with another semiconductor laser light source.
More specifically, controlling only the position of the hologram element in the assembly raises problems such that servo error signals cannot be optimized, or tolerances in the mounting of the laser and the light receiving element, in packaging, and the like are made highly exacting, thereby increasing costs.
Further, the hologram element is often provided with an aberration correction function so as to obtain desirable light converging characteristics on the light receiving element; however, it is difficult to design such a hologram pattern as to perform optimum aberration correction with respect to a plurality of different wavelengths.
Furthermore, the conventional optical pickup has a problem that it is not applicable to a plurality of optical disks of different specs in which different tracking error signals are used, respectively, because only a tracking error signal according to the three-beam method can be detected from either of light beams of the semiconductor laser having a plurality of waveforms.